This invention relates to wastewater treatment, and more particularly to an improved method of treating wastewater in a septic tank.
A septic tank typically provides primary wastewater treatment for domestic wastewater where municipal treatment facilities are unavailable. In a conventionally operated septic tank raw sewage wastewater, having a significant concentration of waste solids, enters one end of the tank and travels through the tank to a discharge end of the tank. In the septic tank, solids separate from the liquid portion of the sewage. Solids having a lower density than the liquid move to the top of the liquid to form a scum layer, and the solids having a higher density than the liquid sink to the bottom of the tank to form a sludge layer, resulting in a relatively clear liquid layer between the scum and the sludge. The liquid portion of the wastewater which exits the discharge end of the tank by means of gravity, a pump, or a siphon, is the septic tank effluent.
The quality of the septic tank effluent primarily determines the appropriate secondary treatment for the wastewater prior to final disposal and is generally measured by the biochemical oxygen demand (BOD), total suspended solids (TSS), and total nitrogen present in the effluent. Since the BOD and total nitrogen concentrations of the septic tank effluent primarily determine the size and kind of the secondary waste treatment facilities, improving the quality of the septic tank effluent quantitatively reduces the secondary treatment necessary to produce a secondary effluent meeting or exceeding post-secondary treatment standards.
Nitrogen in wastewater may exist as organic nitrogen, ammonia, nitrite, and nitrate. The nitrogen present in fresh wastewater is primarily organic nitrogen combined in proteinaceous matter and urea. Decomposition of the organic material by bacteria present in the anaerobic environment of the septic tank readily changes the organic nitrogen to ammonia nitrogen. Thus, in conventionally treated septic tank effluent, nitrogen is present primarily as ammonia nitrogen.
Secondary treatment of septic tank effluent is typically an aerobic treatment which reduces the BOD and TSS of the effluent to acceptable levels prior to discharge of the secondary effluent. In addition, in an aerobic environment, bacteria oxidize ammonia nitrogen to nitrate nitrogen, a process known as nitrification. Thus in a conventional system nitrogen in this secondary effluent is present primarily as nitrate nitrogen. However, it is desirable to minimize the nitrate concentration of the secondary effluent to about 10 mg-N/L, which is the nitrate concentration allowed by the U.S. EPA drinking water standards.
In order to remove nitrogen from the system, nitrate nitrogen must be converted to a readily removable gaseous form of nitrogen. This conversion can be accomplished biologically under anoxic (without oxygen) or anaerobic conditions by several genera of denitrifying bacteria. Denitrifying bacteria are capable of converting nitrate to nitrite, followed by production of nitric oxide (NO), nitrous oxide (N.sub.2 O) and nitrogen gas (N.sub.2). The last three compounds are gaseous products that are released to the atmosphere and are thus removed from the system. The conversion is known as denitrification. However, in order to remove nitrogen from the system, the denitrifying bacteria also require a source of carbon for cell-synthesis. Conventionally treated nitrate-laden liquid does not contain a source of carbon for the denitrifying bacteria since the aerobic process producing the nitrate-laden secondary effluent removes carbon sources by reducing the BOD.
U.S. Pat. No. 4,895,645 to Zorich, Jr. discloses a filter which receives ammonia-laden septic tank effluent in an anaerobic chamber. A portion of the ammonia-laden liquid from the anaerobic chamber is circulated to an aerobic filter where the ammonia is converted to nitrate, and a portion of the ammonia-laden liquid is recirculated back to the anaerobic chamber. The nitrate-laden filtrate from the aerobic filter is returned to the anaerobic chamber to mix with incoming septic tank effluent and undergo denitrification. At certain times, a portion of the liquid from the anaerobic chamber, now laden with ammonia and nitrate, is discharged to a drainfield. Sediment from the anaerobic chamber is returned to the septic tank. Denitrification, or nitrogen removal, in the anaerobic chamber is not efficient because of the large volume of ammonia-laden liquid which is recirculated only through the anaerobic portion of the filter. Denitrification in the septic tank is not efficient because only a small volume of liquid, which also contains both ammonia nitrogen and nitrate nitrogen, is returned to the septic tank. Since no large volume of nitrate-laden liquid is anaerobically treated, liquid discharged from the filter will always contain a fairly high ammonia concentration as well as an appreciable nitrate concentration.
Sandy, et al. in "Enhanced Nitrogen Removal Using a Modified Recirculating Sand Filter (RSF.sup.2)," Proceedings of 5th ASAE Conf. 161 (December 1987), disclose a system which receives septic tank effluent laden with ammonia in a gravel storage zone of a modified recirculating sand filter. A portion of the liquid from the gravel storage zone is distributed over an aerobic filter, where ammonia nitrogen is converted to nitrate nitrogen, and the nitrate-laden aerobic filter effluent is mixed with the ammonia-laden liquid in the storage zone. Ammonia- and nitrate-laden liquid from the storage zone may be discharged or recycled to the septic tank. While some denitrification may take place in the septic tank, the process is inefficient because the nitrate-laden liquid is diluted with ammonia-laden liquid before it is recycled to the septic tank. In addition, the volume of liquid recycled to the septic tank is not as great as the volume of liquid aerobically treated, so all the nitrate-laden liquid is not returned to the septic tank.
Some systems, such as the BIOCLERE.RTM. system marketed by Ekofinn of Federal Way, Wash., discharge septic tank effluent to a baffled sump from which liquid is pumped and distributed over an aerobic filter. Filter effluent is returned to the sump, and the liquid in the sump therefore contains both ammonia nitrogen and nitrate nitrogen. Liquid and solids from the bottom of the sump are periodically returned to the septic tank. However, since the effluent from the aerobic filter is always diluted in the sump with ammonia-contaminated septic tank effluent, it is not possible to attain high volume nitrate reduction and, thus, nitrogen must always be present in the liquid discharged as effluent from the sump.
U.S. Pat. No. 3,567,629 to Ayers, et al. discloses a process for treating sewage. An aerobic sump receives septic tank effluent via a draft tube where the septic tank effluent is mixed with the filter effluent from an aerobic filter. Liquid and solids from the bottom of the sump are redistributed over the aerobic filter, and no recirculation to the septic tank is provided. Liquid is periodically discharged from the sump. Thus, both ammonia nitrogen and nitrate nitrogen will be present in the sump, and little denitrification can take place because the sump is maintained in an aerobic condition.
Thus, what is still needed is an effective method of improving the quality of septic tank effluent by reducing effluent levels of total nitrogen, BOD and suspended solids.